The present invention relates generally to electrohydraulic servovalves and more specifically to such a servovalve having gain compensation to provide operational characteristics desired for specific application to which the valve may be put. For example, only the compensation may be such as to linearize the flow characteristic thereof including substantially eliminating the deadband resulting from a substantial overlap condition and other non-linear flow characteristic inherent in a specific type of valve.
Electrohydraulic servovalves are well known in the prior art and are used in many applications. As is well known to those skilled in the art, such a servovalve converts electrical control signals into output hydraulic signals for application to a fluid motor for use in various applications. In such devices, it is usually desirable to have a flow gain characteristic which is substantially linear with respect to the application of input control signals. It is also desirable to have a minimum amount of leakage when the valve is in its null position. When the valve has these operating characteristics, a very small amount of applied input signal will initiate movement of the valve thereby opening the flow ports to allow flow of fluid therethrough. The deadband is eliminated typically by providing rectangular flow ports with precisely fitted lands which open and close those ports. There is thus provided a line-to-line lap fit of lands on a spool valve to rectangular flow ports which provide the elimination of the undesirable deadband as the spool passes through its null region. However, such line-to-line lap fit is also synonymous with high quiescent spool leakage. To overcome the spool leakage, one needs to extend the spool so that there is sufficient overlap between the spool and the rectangular flow port edge. When such occurs the leakage is reduced to substantially zero. However, under such conditions, the undesirable deadband is introduced.
In addition to the foregoing, to manufacture rectangular ports and provide line-to-line spool lap is extremely expensive requiring machining operations such as EDM as well as flow grinding and lapping to match the spool and flow port edges precisely.
In an attempt to reduce the cost of manufacture of such electrohydraulic servovalves, flow ports of different shapes than rectangular have been utilized from time to time. One such non-rectangular flow port is a round flow port. Such round flow ports are relatively easy to manufacture and do not require expensive machining operations. However, such flow ports with sufficient overlap to minimize leakage at the null position provide a highly non-linear flow gain curve of the type as is illustrated in FIG. 1. In FIG. 1, the flow is plotted on the ordinate with spool position on the abscissa (spool position being commensurate with input signal). As can be seen, there is a substantial deadband (effectively zero flow) about the center or null position of the spool. Even when flow does occur, it is relatively small compared to spool position since a round flow port when initially opened provides only a minimum amount of flow.
As a result, electrohydraulic servovalves having round holes for flow ports have, in the prior art, not been acceptable low cost alternatives to the rectangular flow ports with sufficient overlap to compensate for leakage.
In the prior art, to compensate for the deadband created by the substantial overlap there has been provided a control circuit which is used to modify the input signal causing the spool to shift rapidly out of the deadband range and thereby more closely approximate a linear flow gain. Such a device is shown in U.S. Pat. No. 3,821,625. As is therein disclosed, an operational amplifier has a near infinite gain so that an incrementally small input signal saturates the amplifier to produce a fixed maximum output voltage. This signal is coupled with the input control signal to sufficiently shift the valve to eliminate the spool overlap which causes the deadband. Although such compensation would assist in eliminating the deadband region as shown in FIG. 1, it would not eliminate other non-linearity characteristics of the flow versus spool position curve such as those created by round flow ports.
U.S. Pat. No. 4,513,782 discloses an electrohydraulic servovalve device designed to provide a desired linear flow in response to all possible electrical signals applied thereto. To accomplish this, the valve is tested at the time it is manufactured and deviations from linear are noted. An EPROM is then programmed to alter the applied electrical signals to overcome the deficiencies of that specific valve and provide a valve having a linear response. The EPROM becomes a permanent part of the valve.